When preparing communication signals for transmission, amplifier circuits are used to increase transmitted signal output power. These amplifier circuits impart distortion on the signals amplified. Primarily, this distortion takes the form of changes in amplifier amplification as a function of input signal amplitude. These amplification changes affect both the in-phase and quadrature-phase components of the signal amplified. Such distortion impacts both signal modulation accuracy and spectrum emissions. Modulation accuracy must be maintained to ensure link clarity. Spectrum emissions must be maintained to meet regulatory body mandated compliance. Amplifier distortion therefore must be maintained within the limits of modulation accuracy and spectrum emissions.
Amplifier linearity can be achieved by various means, with each means bearing different performance, complexity, and cost challenges. Predistortion is one such approach. Several predistortion methods have been attempted to improve amplifier linearity with various levels of success. For example, simple, in line, RF circuit predistorters have been constructed which provide modest linearity improvement. Such circuits generally fail to sufficiently improve amplifier linearity with respect to modulation accuracy or spectrum emissions requirements particularly when used in conjunction with Class AB amplifiers.
A more sophisticated approach employs predistorters based on complex modulation of the input RF signal, as a function of input signal amplitude. Such predistorters require time delay in the RF signal path to provide time correlation with the RF amplitude detection, predistortion function application, and signal modulation. When such predistortion methods are used, the time delay of the amplitude detection, predistortion function application, and RF signal modulation path should be kept to a minimum, in order to keep RF time delay circuits manageable in size and cost. Of these three circuits in this path, predistortion function application circuits are the most challenging. First, the function generated must include complex components (in-phase and quadrature-phase). Second, the function must be a near inverse of the imparted amplifier distortion in order to linearize the amplifier to modulation accuracy and spectrum emissions requirement limits. Third, the function must adapt to track changes in the amplifier based on amplifier supply voltage, temperature, aging, etc.
Previous designs have met the above noted three predistortion function generation goals with various levels of success. Some use analog circuits to create the function while making little or no effort in tracking amplifier changes. These attempts generally provide slightly better performance than simple, in line, RF predistorters but with a significant increase in circuit design complexity. Others use digital means to create, adapt, and apply the predistortion function to the predistortion modulator. While successful, these digital methods greatly increase design complexity and cost. When creating the predistortion modulation signal using digital means, Nyquist sampling requirements must be met. Meeting Nyquist requirements means using high speed analog-to-digital and digital-to-analog conversion circuits. Signal processing must be performed at Nyquist based rates. Also, anti-aliasing and reconstruction filtering must be used prior to sampling the amplitude signal, and after creating the function based predistortion modulation signal respectively. Finally, all the circuit complexity caused by using the digital approach increases function generation delay. Delay circuits can become quite large and costly.
Accordingly a need presently exists for a more efficient approach to implementation of predistortion linearization of power amplifiers.